This invention relates to an optical scanning system for scanning the information content of an original document and, more particularly, to a scanning system which employs a screw type driving means to simultaneously move a plurality of scanning elements at different rates of speed.
In the copying art, relatively compact copiers such as the Xerox 3100 have been made possible by using dual rate scanning techniques to shorten the optical path. Typically, these systems, as shown in U.S. Pat. Nos. 3,832,057 and 4,155,641, utilize scanning elements which are mounted on carriages and driven horizontally beneath a platen to scan successive incremental areas of a document on the platen. Each scan carriage, typically driven by a pulley-gear arrangement operating off the photoreceptor drive, travels at a different rate of speed (1:1/2) so as to maintain the required total conjugate length. The scanned images are then projected through a lens onto the photoreceptor surface.
These dual rate optical scan systems have several attendant disadvantages. They require a complex cable drive system with difficult-to-achieve pulley and gear ratios. Cable tensioning must be accurately set up and maintained. Acceleration and deceleration effects contribute to unwanted carriage side movements resulting in skipped or blurred images.
There are additional prior art systems which would benefit from a more efficient dual rate scan system. For example, in raster scanning systems, the original document is scanned and the line images projected onto image arrays, typically CCD elements. Each of the arrays produce a stream of discrete video signals representative of the image being scanned.
In this type of screw system, extremely precise control of the scan mechanism is required because of the narrow exposure "window" of the image arrays. The pulley system described above is not precise enough to realize the exacting screw requirements. One alternative known in the art is to mount the scan carriage on a lead screw as is disclosed in U.S. Pat. No. 4,149,090 and in co-pending U.S. application Ser. No. 084,202, filed Oct. 12, 1979, now abandoned, assigned to the same assignee as the present application. The pitch of the lead screw can be machined to provide a precisely defined rate of travel of the scan carriage. This solution, however, requires the simultaneous movement of additional optical elements; e.g. folding mirrors, lens, beamsplitters.